11.1 Mendel and the Garden Pea Heredity: the tendency for traits to be passed from parent to offspring heritable features: characters traits are alternative forms of a character Genes Alleles loci Gregor Mendel solved the puzzle of heredity Mendel performed experiments with garden peas Why study the garden pea? many varieties/ easily distinguishable traits that can be quantified small, easy to grow, and produce large numbers of offspring quickly their reproductive organs easily manipulated so that pollination can be controlled self- fertilize Mendel had a specific experimental design st : establish true- breeding varieties Plants self- fertilize for several generations, (each variety had only 1 type of trait) pure lines: P generation Mating P generation è F 1 F 1 generation self- fertilize èf 2 generation 1 st Fig 11.3 How Mendel conducted his experiments 11.2 What Mendel Observed for each pair of contrasting varieties that he crossed, one of the traits disappeared in the F1 gen but reappeared in the F2 gen trait expressed in F1 gen: dominant trait not expressed in F1 gen: recessive trait 11.2 What Mendel Observed 1
Mendel counted # of each type of plant in the F2 generation 3/4of F2 individuals expressed the dominant trait while 1/4 expressed the recessive trait the dominant:recessive ratio among the F2 plants was always close to 3:1 Table 11.1 Seven Characters Mendel Studied in his Experiments 11.2 What Mendel Observed recessive trait hidden? in F 1 gen & not expressed He allowed the F 2 to self- fertilize and form the F 3 generation he found that one- fourth of the plants from the F 2 that were recessive were true- breeding in the F 3 he found that of the three- fourths of the plants from the F 2 only one- third were true breeding in the F 3 the remaining half sh owed both traits 11.2 What Mendel Observed He determined that the ratio of 3:1 ratio that he observed in the F 2 generation was in fact a disguised 1:2:1 ratio 1: true breeding dominant 2: not true breeding 1: true breeding recessive Fig 11.5 The F 2 gen is a disguised 1:2:1 ratio 2
Mendel s 5 Hypothesis Theory Hypothesis 1 parents do not transmit traits directly to offspring parents transmit information about the trait in the form of what Mendel called factors (Now called genes) Hypothesis 2 each parent contains 2 copies of factor governing each trait the 2 copies of the factor may or may not be same Homozygous: two of the same copies Heterozygous: two different copies: Hypothesis 3 alternative forms of a factor lead to alt. traits Alleles: alternative forms of a factor appearance is determined by the alleles a plant receives from its parents ( genotype) expression of the alleles =appearance ( phenotype) Hypothesis 4: 2 alleles do not affect each other Hypothesis 5 presence of allele does not ensure expression of trait in heterozygotes, only dominant allele is expressed Fig 11.6 Alternative alleles of genes are located on homologous chromosomes Table 11.2 Some Dominant and Recessive Traits in Humans 11.3 Mendel Proposes a Theory By convent ion, genetic traits are assigned a letter symbol referring to their more common form dominant traits: capitalized recessive trait: lower- case Ie: flower color in peas is represented as follows P signifies purple p signifies white 3
Fig 11.7 Punnett squar e analysis Figure 11.8 How Mendel analyzed flower color Testcross: determine the genotype of unknown individuals in the F 2 gen unknown individual is crossed with a homozygous recessive individual if the unknown is homozygous, then all of the offspring will express dominant traits if the unknown is heterozygous, then one- half of the offspring will express recessive traits Figure 11.9 How Mendel used the testcross to detect heterozygotes Mendel s 1st Law: Segregation the two alleles of a trait separate from m each other during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy 11.4 Mendel s Laws Mendel also investigated the inheritance pattern for more than one factor when crossing individuals who ar e true- breeding for 2 different characters, the F1 individual that results is a dihybrid BBFf X BbFF after the dihybrid individuals self- fertilize, there are 16 possible genotypes of offspring Figure 11.10 Analysis of a dihybrid cross Conclusion: the inheritance of one trait does not influence the inheritance of the other trait Mendel s 2nd Law: INDEPENDENT ASSORTMENT genes located on different chromosomes are inherited independently of one another 4
Figure 11.11 The journey from DNA to phenotype Some T raits Don t Show Mendelian Inheritance Often the expression of phenotype is not straightforward Continuous variation characters can show a range of small differences when multiple genes act jointly to influence a character this type of inheritance is cal led polygenic Figure 11.12 Height is a continuously varying character Some Traits Don t Show Mendelian Inheritance Pleiotropic effects an allele that has more than one effect on a phenotype is considered pleiotropic these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle- cell anemia Figure 11.13 Pleiotropic effects of the cystic fibrosis gene 11.6 Why Some Traits Don t Show Mendelian Inheritance Incomplete dominance not all alternative alleles are dominant or recessive in heterozygotes some alleles exhibit incomplete dominance: produce a heterozygous phenotype (intermediate between 2 parents) Figure 11.14 Incomplete dominance 5
11.6Environmental effects Expression of some alleles depends on environment Ie: some alleles are heat- sensitive arctic foxes only produce fur pigment when temperatures are warm Fig 11.15 Environmental effects on an allele Codominance: often, in heterozygotes, there is not a dominant allele but, instead, both alleles are expressed these alleles are said to be codominant Chromosomal theory of inheritance was first proposed in 1902 by Walter Sutton supported by several pieces of evidence reproduction involves union of only eggs & sperm each gamete contains only 1 copy of the genetic information since sperm have little cytoplasm, the material contributed must reside in the nucleus chromosomes both segregate and assort independently during meiosis Linkage: the tendency of close ose-together genes to segregate together Fig 11.22 Linkage the further two genes are from each other on the same chromosome, the more likely crossing over is to occur between them (this would lead to independent segregation) the closer that two genes are to each other on the same chromosome, the less likely that crossing over will occur between them these genes almost always segregate together and would, thus, be inherited together 6
11.8 Human Chromosomes Each human somatic cell normally has 46 chromos omes, (23 pairs) #1-22 pairs are perfectly matched in both males and females and are called autosomes #23 pair are the sex chromosomes females are designated XX while males are designated XY the genes on the Y chromosome determine maleness Sometimes er rors occur during meiosis Nondisjunction: failure of chromosome to separate during meiosis I or meiosis II leads to aneuploidy: abnl chromosome # most result in failure to develop/early death before adulthood extra copy of chromosome 21 or, more rarely, chromosome c 22 can survive to adulthood delayed development and mental impairment Down syndrome: extra copy of # 21 Fig 11.23 Nondisjunction in anaphase I ( Down Syndrome) Nondisjunction may also affect the sex chromosomes nondisjunction of the X chromoso me creates three possible viable conditions XXX female: usually taller than average but other symptoms vary XXY male (Klinefelter syndrome): : sterile male with many female characteristics and diminished mental capacity XO female (Turner syndrome): : steril e female with webbed neck and diminished stature Figure 11.26 Nondisjunction of the X chromosome 7
Nondisjunction of the Y chromosome also occurs in such cases, YY gametes are formed, leading to XYY males these males are fertile a nd of normal appearance Accidental changes in genes are called mutations occur only rarely and almost always result in recessive alleles not eliminated from the population because they are not usually expressed in most individuals (heterozygotes) When mutant m alleles produce harmful effects: genetic disorders Table 11.3 Some Genetic Disorders 11.9 The Role of Mutations in Human Heredity To study human heredity, scientists examine crosses that have already been made family trees or pedigree often one can c determine whether a trait is sex- linked or autosomal and whether the trait s phenotype is dominant or recessive for example, hemophilia is a sex- linked trait Figure 11.27 A general pedigree Fig 11.28 Royal hemophilia pedigree Sickle- cell anemia: recessive hereditary disorder Affected: homozygous recessive carry a mutated gene that produces a defective version of hemoglobin hgb sticks together inappropriately and produces a stiff red blood cell with a sickle- shape 8
the cells cannot move through the t blood vessels easily and tends to clot this causes sufferers to have intermittent illness and shortened life spans Heterozygous individuals have some of their TBC s sickled when O2 levels become low the sickle- cell allele more frequent among people in malarial regions the presence of the allele increases resistance to malaria infection Sickle- cell Anemia Tay- Sachs disease is another disease caused by a recessive allele it is an incurable disorder in which the brain deteriorates sufferers rarely live beyond five years of age Huntington s disease is a genetic disorder caused by a dominant allele it causes progressive deterioration of brain cells every individual who carries the allele expresses the disorder but most persons do not know they are affected until they are more than 30 years old Figure 11.33 Huntington s disease is a dominant genetic disorder 11.10 Genetic Counseling and Therapy Genetic counseling is the process of identifying parents at risk of producing children with genetic defects and of assessing the genetic state of early embryos 11.10 Genetic Counseling and Therapy identify high- risk pregnancie & the chances of both parents being heterozygote carriers of an allele for a recessive genetic disorder high- risk also identified when the mothers are > 35 years old Genetic counselors also utilize genetic screening Amniocentesis: amniotic fluid is sampled and isolated fetal 9
cells are then grown in culture and analyzed chorionic villus sampling: fetal cells from the chorion in the placenta are removed for analysis Figure 11.34 Amniocentesis Genetic counselors look at 3 things from the cell cultures obtained from either amniocentesis or chorionic villus sampling chromosomal karyotype: analysis can reveal aneuploidy or gross chromosomal altera tions enzyme activity: in some cases, it is possible to test directly for the proper functioning of enzymes associated with genetic disorders genetic markers: test for the presence of mutations at the same place on chromosomes where disorder- causing muta tions are found DNA screening: most recent form of genetic counseling screens DNA for the presence of key genes utilizing information from the Human Genome Project, the DNA of patients is assessed for copies of genes that lead to hereditary disorders, in addition, parents conceiving by in vitro fertilization (i.e., test- tube babies) can screen zygotes for potential genetic anomalies this procedure is called preimplantation screening Figure 11.36 Preimplantation genetic diagnosis The End of Chapter 11 On to chapter 12: DNA, Protein Synthesis 10